A ban on the entry to Beijing of high-emission motor vehicles came into force on Tuesday. It is the latest step by Beijing to address growing concerns about air pollution as the number of cars in the capital now totals 3.7 million.
The ban, issued by the Ministry of Environmental Protection on July 28, forbids petrol vehicles below National Emission Standard I to travel along or inside the city's Fifth Ring Road.
It also says diesel-driven vehicles will have to at least comply with National Emission Standard III before they can operate in the same area.
Standard I, which is equivalent to Euro I standard, allows an average petrol sedan to emit a maximum of 2.7 grams of carbon monoxide a kilometer among its other exhausts, whereas Standard IV requires less than 1 gram of carbon monoxide and 0.08 gram of nitrogen oxide a kilometer.
Beijing's regulations on vehicle exhaust emissions, which adopt European standards, are tougher than US federal standards.
The US's Tier 2 standard requires vehicles to emit less than 2.125 grams of carbon monoxide and 0.25 gram a kilometer.
The ministry says the area of the ban will extend to the Sixth Ring Road, the city's outermost highway loop, from October 1 when China celebrates its 60th anniversary.